Nuclear medical imaging is a useful technique that is applied in a variety of diagnostic contexts and medical specialties. In one type of nuclear medical imaging known as Single Photon Emission Computed Tomography (SPECT), the primary imaging task is to accurately determine and depict the spatial distribution of a radioactive isotope (radioisotope) used as a tracer (radiotracer) in the imaged object. A radiotracer may also be referred to as a radiopharmaceutical, as it is a pharmaceutical labeled with a radioactive isotope. Typically, the radiotracer travels to a target within the patient's body, and an attached radioactive atom emits gamma ray photons as it undergoes nuclear decay.
A gamma camera (also known as an Anger camera or scintillation camera) has one or more detectors located near the patient for detecting emitted gamma rays that have traveled through a portion of the patient's body. For example, FIG. 1 shows a gamma camera 100 having a detector 105 (e.g., a SPECT detector that detects gamma photons) mounted on a gantry 103 that is secured by a stand 104. Gantry 103 carries a motion guide 106, which in turn carries detector 105. Although one detector is shown in FIG. 1, other configurations are possible, e.g., two detectors 105a and 105b mounted to gantry 103 as shown in the perspective view of FIG. 2. By mounting one or more detectors on gantry 103, the detector(s) can be positioned in various orientations (e.g., relative to the surface of the earth) as the gantry is rotated. For example, in the head-on view of FIG. 1, detector 105 is shown positioned directly above the patient 101 lying on a table top 102, and this orientation may correspond to a gantry rotation angle of 0 degrees (other conventions for defining the rotation angle can be used). As gantry 103 rotates, detector 105 moves along a circular path corresponding to respective gantry rotational orientations. The capability for varying the gantry rotational orientation enables the patient 101 to be imaged from various angles.
For SPECT imaging, gamma camera data are acquired from various view angles or projections and reconstructed in various planes in accordance with the principles of three-dimensional (3D) tomography. A gamma camera used for SPECT is also referred to as a SPECT camera. Gamma cameras for SPECT are described in more detail in Sayed, M., “Quality Control in Nuclear Medicine II. Planar and Single Photon Emission Computed Tomographic Gamma Camera Apparatus,” Turkish Journal of Nuclear Medicine 6(3): 185-189 (1997), the entire contents of which are hereby incorporated by reference herein.
Photomultiplier tubes (PMTs) in a gamma camera, which detect and amplify scintillation events, are very sensitive to magnetic fields. One way to address this sensitivity to magnetic fields is to shield individual PMTs with metallic structures. But, such metallic structures that are conventionally used can only extend to the face of a PMT's photocathode, which limits the magnetic shielding effectiveness for the region between the photocathode and one of the electrodes (dynode) of the PMT. Thus, PMTs are susceptible to variation in results due to magnetic fields, despite existing shielding mechanisms.